It's plextors powerrec that kicks in.
Arrita is Ritek B-brand and sells mostly B-quality disc's.
Now the plextor drive will recognise the disc as a ricoh disc cappable of 8x burning however when it founds out that for some reason there might show up problems it will pull the speed down.
Switching an controlling costs time.
Plextors quality control works much sooner when set at maximum for that reason it might even be as critical to put it down to 2,4x.
Second burning Ricoh media at 8x isn't a very good thing if you read c't (germany).
No problems with MCC's 4x media which burns very good at 8x.(However the quality for that one is also better at 4x but the difference is much smaller as with ricoh media)